1. Field of the Invention
The present invention relates to an electric motor-driven power steering apparatus and, more particularly to an electric motor-driven power steering apparatus suitably applicable to a vehicle steering apparatus such as an automobile steering apparatus, a battery type fork lift steering apparatus etc..
The electric motor-driven power steering control apparatus includes a steering power assist control means which controls a steering power assist for a steering wheel by use of a motor. The steering power assist control means of the present invention comprises a main steering power assist control means and an auxiliary steering power assist control means.
2. Prior Art
A conventional electric motor-driven power steering apparatus in, for example in U.S. Pat. No. 4,614,248, includes a steering power assist control means. The steering power assist control means comprises only single steering power assist control means.
The conventional electric motor-driven power steering apparatus produces an assist power for the steering operation by energizing a motor through the single steering power assist control means and a motor drive means according to a steering torque input signal such as a torsional torque input signal.
The above stated conventional electric motor-driven power steering apparatus does not provide a protect means to protect against a failure of the single steering power assist control means.
Namely, when the single steering power assist control means of the conventional electric motor-driven power steering apparatus fails while the steering wheel is being turned and the steering power assist is being generated by the motor, the steering power assist will suddenly change, producing an unexpected steering state for the driver. This is very dangerous.
Especially when the automobile is cornering at a high speed, a sudden loss of the steering power assist will make the steering wheel heavy instantly, with a possibility of the automobile running out of control. Also, when the single steering power assist control means fails and an automobile which was running in a straight line is going to make a turn, the driver may not be able to cope with the unexpected situation.